Chemical and Physical Aspects of Macroemulsions Stabilized by Interfacial Colloids

A Review on Use of Emulsified Acids for Matrix Acidizing in Carbonate Reservoirs

Almost 60% of oil and 40% of gas reserves worldwide are contained in carbonate reservoirs where acidizing stimulation is more challenging compared to sandstone reservoirs. Utilization of emulsified acids in matrix acidizing operations has been the most effective technique for more than half a century. This is due to the colloidal system's ability to generate deep, narrow conduits toward production zones under controlled retarded reactivity with the rock surface, along with the excellent sweep efficiency and corrosion inhibition of the well equipment. This Review attempts to review the various kinds of emulsified acids that are used for matrix acidizing of carbonate formations. The Review is trying to trace the innovations that have, gradually, been applied for enhancing the performance of emulsified acids for a variety of conditions, their limitations, and the developmental solutions such hybrid emulsifiers and pickering emulsions. In addition, the Review also discusses the parameters, characteristics, and measuring techniques required for the successful synthesis of a stable and quality emulsion while considering the environmental concerns raised toward the application of an emulsified acid system. From the reviewed publications, it can be summarized that macroemulsions are best suited for matrix acidizing applications over microemulsions due to low emulsifier concentrations and high acid volume retention; similarly, water in oil emulsions provide better retardation in a comparison to oil in water emulsions. The small droplet size of the emulsion yields high viscosity and stability. The compositional balance between each component present in the acidizing system is a crucial factor for optimum performance. Moreover, for future practice, much focus is required to design emulsified acids as ecofriendly systems that can leave the least amount of toxicity during and after implementation.

A Robust Switchable Oil-In-Water Emulsion Stabilized by Electrostatic Repulsions between Surfactant and Similarly Charged Carbon Dots

How to control the stability of oil-in-water (O/W) emulsions is one of the main topics for scientists working in colloidal systems. Recently, carbon dots (CDs) have received great interest as smart materials because of their excellent physicochemical properties and versatile applications. Herein, for the first time, advanced and switchable O/W emulsions are presented that are stabilized by the synergistic effect of cationic surfactant cetyltrimethylammonium bromide CTAB (emulsifier) and similarly charged CDs (stabilizer). In the formulated emulsion, the cationic surfactant molecules are adsorbed at the oil and water interface to decrease the interfacial tension and enrich the drops with a positive charge to ensure intensive electrostatic repulsions among them. On the contrary, cationic CDs are distributed in the water phase among the droplets to reduce the water secretion and prevent flocculation and droplet coalescence. The stabilizing effect is found to be universal for emulsions of a range of oil phases. Furthermore, the formulated emulsion is found to be switchable between "stable" and "unstable" modes by adding an equivalent of anionic surfactant sodium dodecyl benzene sulphonate (SDBS). The stabilized and switchable O/W emulsions are believed to have wide practical applications in water purification, pharmaceuticals, protein recognition, as well as catalysis.

Switchable Oil-in-Water Emulsions Stabilized by Like-Charged Surfactants and Particles at Very Low Concentrations

A novel CO₂/N₂ switchable n-decane-in-water emulsion was prepared, which is stabilized by a CO₂/N₂ switchable surfactant [ N'-dodecyl- N, N-dimethylacetamidine (DDMA)] in cationic form in combination with positively charged alumina nanoparticles at concentrations as low as 0.01 mM and 0.001 wt %, respectively. The particles do not adsorb at the oil-water interface but remain dispersed in the aqueous phase between surfactant-coated droplets. A critical zeta potential of the particles of ca. +18 mV is necessary for the stabilization of the novel emulsions, suggesting that the electrical double-layer repulsions between particles and between particles and oil droplets are responsible for their stability. By bubbling N₂ into the emulsions, demulsification occurs following transformation of DDMA molecules from the surface-active cationic form to the surface-inactive neutral form and desorption from the oil-water interface. Bubbling CO₂ into the demulsified mixtures, cationic DDMA molecules are re-formed, which adsorb to the droplet interfaces, ensuring stable emulsions after homogenization. Compared with Pickering emulsions and traditional emulsions, the amount of switchable surfactant and number of like-charged particles required for stabilization are significantly reduced, which is economically and environmentally benign for practical applications.

O/W emulsions stabilised by both low molecular weight surfactants and colloidal particles: The effect of surfactant type and concentration

The stability against coalescence of O/W emulsions in the presence of both surfactants and colloidal particles was investigated. In particular the effect of the surfactant type and concentration in these emulsifier mixtures on the O/W emulsions' stability was studied. Two types of surfactants were selected; those that have the ability to stabilise O/W emulsions on their own (O/W surfactants) and those that cannot (W/O surfactants). Tween 60 and Sodium Caseinate were selected as the O/W surfactants and lecithin as the W/O surfactant. Oil-in-water emulsions prepared with both particles and any of the three surfactants were stable against coalescence but, depending on the type of surfactant, the behaviour of the systems was found to depend on surfactant concentration. The droplet sizes of emulsions stabilised by mixed emulsifier systems containing low concentrations of O/W surfactants (Tween 60 or Sodium Caseinate) were smaller than those solely stabilised by either the surfactant or particles alone. At intermediate O/W surfactants concentrations, the droplet sizes of the emulsions increased. Further increases in the O/W surfactants' concentration, resulted in the complete removal of particles from the interface with the system now behaving as a surfactant-only stabilised emulsion. The behaviour of emulsions stabilised by emulsifier mixtures containing W/O surfactants was not dependent on the concentration of surfactant: no removal of particles was observed.

Aqueous foams stabilized by in situ surface activation of CaCO3 nanoparticles via adsorption of anionic surfactant

The in situ surface activation of unmodified CaCO(3) nanoparticles by interaction with surfactant in aqueous media has been studied, and the impact of this on the foamability and foam stability of aqueous dispersions was assessed. Using complementary experiments including measurement of particle zeta potentials, adsorption isotherms of surfactant, air-water surface tensions, and relevant contact angles, the mechanism of this activation was revealed. The results show that the non-surface-active CaCO(3) nanoparticles cannot be surface activated by interaction with cationic or nonionic surfactants but can be surface activated by interaction with anionic surfactants such as SDS and AOT, leading to a synergistic effect in both foamability and foam stability. The electrostatic interaction between the positive charges on particle surfaces and the negative charges of anionic surfactant headgroups results in monolayer adsorption of the surfactant at the particle-water interface and transforms the particles from hydrophilic to partially hydrophobic such that particles become surface active and stabilize bubbles. SDS is a more efficient surfactant for this surface activation than AOT. Possible reasons for this difference are suggested.

Effects of surfactant structure on the phase inversion of emulsions stabilized by mixtures of silica nanoparticles and cationic surfactant

Silica nanoparticles without any surface modification are not surface active at the toluene-water interface due to their extreme hydrophilicity but can be surface activated in situ by adsorbing cationic surfactant from water. This work investigates the effects of the molecular structure of water-soluble cationic surfactant on the surface activation of the nanoparticles by emulsion characterization, adsorption and zeta potential measurements, dispersion stability experiments, and determination of relevant contact angles. The results show that an adsorbed cationic surfactant monolayer on particle surfaces is responsible for the wettability modification of the particles. In the presence of a trace amount of cationic surfactant, the hydrophobicity of the particles increases, leading to the formation of stable oil-in-water O/W(1) emulsions. At high surfactant concentration (>cmc) the particle surface is retransformed to hydrophilic due to double-layer or hemimicelle formation, and the concentration of the free surfactant in the aqueous phase is high enough to stabilize emulsions alone. O/W(2) emulsions, probably costabilized by free surfactant and particles, are then formed. The monolayer adsorption seems to be charge-site dependent. Thus, using single-chain trimethylammonium bromide surfactants or a double-head gemini cationic surfactant, the hydrophobicity of the particles achieved is not sufficient to stabilize water-in-oil (W/O) emulsions, and no phase inversion is induced. However, using a double-chain cationic surfactant, the chain density on the particle surfaces endows them with a hydrophobicity high enough to stabilize W/O emulsions, and double phase inversion, O/W(1) --> W/O --> O/W(2), can then be achieved by increasing the surfactant concentration.

Studies on the kinetics of growth of silver nanoparticles in different surfactant solutions

Silver nanoparticles were prepared in aqueous silver nitrate solution using hydrazine as reducing agents in presence of two ionic surfactants (cetyltrimethylammonium bromide; CTAB and sodium dodecyl sulfate; SDS) and one non-ionic surfactant (Triton X-100). The reaction rate was determined spectrophotometrically. The nature of the head group of these surfactants is responsible for the formation of stable, yellow and transparent silver sol. For a certain reaction time, i.e., 20 min, the absorbance of reaction mixture first increased until it reached a maximum, then decreased with [hydrazine]. The reaction follows first-order kinetics with respect to each in [hydrazine] and [Ag(+)]. The results suggest formation of a complex between silver(I) and hydrazine, decomposes in a rate-determining step, leading in the formation of a free radical, which again reacts with the silver(I) in a subsequent fast step to yield the products. The transmission electron microscopic (TEM) images show that CTAB stabilized silver nanoparticles are spherical and of uniform particle size, and the average particle size is about 15 nm.

Formation and characterization of surfactant stabilized silver nanoparticles: a kinetic study

Kinetic data for the silver nitrate-ascorbic acid redox system in presence of three surfactants (cationic, anionic and nonionic) are reported. Conventional spectrophotometric method was used to monitor the formation of surfactant stabilized nanosize silver particles during the reduction of silver nitrate by ascorbic acid. The size of the particles was determined with the help of transmission electron microscope. It was found that formation of stable perfect transparent silver sol and size of the particles depend upon the nature of the head group of the surfactants, i.e., cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulphate (SDS) and Triton X-100. The silver nanoparticles are spherical and of uniform particle size, and the average particle size is about 10 and 50 nm, respectively, for SDS and CTAB. For a certain reaction time, i.e., 30 min, the absorbance of reaction mixture first increased until it reached a maximum, then decreased with [ascorbic acid]. The reaction follows a fractional-order kinetics with respect to [ascorbic acid] in presence of CTAB. On the basis of various observations, the most plausible mechanism is proposed for the formation of silver nanoparticles.

Factors controlling the formation and stability of air bubbles stabilized by partially hydrophobic silica nanoparticles

Air bubbles have been formed using partially hydrophobic silica nanoparticles as the stabilizer. The particles were of primary particle size 20 nm, chemically treated to different degrees with dichlorodimethylsilane to render them partially hydrophobic. Above a certain bubble size range (typically 80-microm diameter), the bubbles seemed to be almost indefinitely stable, while for any size above 20 microm their stability against disproportionation is far better than bubbles stabilized by any protein film investigated in previous studies. A possible theoretical justification for this observation is presented. Bubbles could be formed by shaking water with the particles, but a much higher volume fraction of bubbles was obtained by pressurizing the aqueous phase to 5 atm overnight followed by suddenly releasing the pressure to nucleate bubbles within the silica dispersion. Sonicating the silica dispersion before nucleation also gave more bubbles, which were also found to be more stable. There appeared to be an optimum degree of surface hydrophobicity that gave maximum foamability and foam stability, where around 20-33% of the silanol groups on the silica surface had been converted to dimethylsilane groups. However, a sharp increase in stability occurred when between 1.8 and 2 mol dm(-3) NaCl was also included in the aqueous phase. The change in stability due to inclusion of salt can be rationalized in terms of changes occurring in the value of the particle contact angle. The effects of increasing sonication and an optimum surface chemical treatment can be explained by the need to make the particles sufficiently hydrophobic so that they adsorb strongly enough, while at the same time minimizing their tendency to aggregate in the bulk aqueous phase, which hinders their adsorption. Furthermore, confocal laser scanning microscopy of the bubble dispersions suggests that a large volume fraction of stable bubbles is only formed when the particles adsorbed to the bubbles are also part of a spanning silica particle network in the bulk aqueous solution, forming a weak gel with a finite yield stress.

Some general features of limited coalescence in solid-stabilized emulsions

We produce direct and inverse emulsions stabilized by solid mineral particles. If the total amount of particles is initially insufficient to fully cover the oil-water interfaces, the emulsion droplets coalesce such that the total interfacial area between oil and water is progressively reduced. Since it is likely that the particles are irreversibly adsorbed, the degree of surface coverage by them increases until coalescence is halted. We follow the rate of droplet coalescence from the initial fragmented state to the saturated situation. Unlike surfactant-stabilized emulsions, the coalescence frequency depends on time and particle concentration. Both the transient and final droplet size distributions are relatively narrow and we obtain a linear relation between the inverse average droplet diameter and the total amount of solid particles, with a slope that depends on the mixing intensity. The phenomenology is independent of the mixing type and of the droplet volume fraction allowing the fabrication of both direct and inverse emulsion with average droplet sizes ranging from micron to millimetre.